A Frame of Reference can be regarded as a ruler with identical clocks (indicated by the red boxes) at each and every point. An observer standing at point x reading time t at his/her specific clock is indicated by the ordered pair, [x,t]. These are the spacetime coordinates of the observer.

In relativistic physics, time is not universal. Even though all clocks at rest are identical and two observers at x=0 may synchronize their watches at time t=0, their clock readings will diverge (un-synchronize) if the observers are in motion relative to each other.

In this animation, two observers on separate frames are traveling at speed 0.6c relative to each other. If we stand on one frame and observe the moving frame, then we see the moving clocks running at 80% the rate of our own (stationary) clocks. Moreover, we perceive lengths of rulers that move in the direction of motion to contract to 80% the lengths of identical stationary rulers.